Use stat_malus when decreasing stats

This patch applies stat_bonus when increasing and stat_malus when decreasing stats.

src/search.cpp

@@ -83,6 +83,9 @@ constexpr int futility_move_count(bool improving, Depth depth) {
 // History and stats update bonus, based on depth
 int stat_bonus(Depth d) { return std::min(392 * d - 467, 1858); }
 
+// History and stats update malus, based on depth
+int stat_malus(Depth d) { return std::min(392 * d - 467, 1858); }
+
 // Add a small random component to draw evaluations to avoid 3-fold blindness
 Value value_draw(const Thread* thisThread) {
     return VALUE_DRAW - 1 + Value(thisThread->nodes & 0x2);
@@ -566,12 +569,12 @@ Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, boo
                 // the previous ply (~0 Elo on STC, ~2 Elo on LTC).
                 if (prevSq != SQ_NONE && (ss - 1)->moveCount <= 2 && !priorCapture)
                     update_continuation_histories(ss - 1, pos.piece_on(prevSq), prevSq,
-                                                  -stat_bonus(depth + 1));
+                                                  -stat_malus(depth + 1));
             }
             // Penalty for a quiet ttMove that fails low (~1 Elo)
             else if (!ttCapture)
             {
-                int penalty = -stat_bonus(depth);
+                int penalty = -stat_malus(depth);
                 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
                 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
             }
@@ -1060,7 +1063,7 @@ Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, boo
                 if (newDepth > d)
                     value = -search<NonPV>(pos, ss + 1, -(alpha + 1), -alpha, newDepth, !cutNode);
 
-                int bonus = value <= alpha ? -stat_bonus(newDepth)
+                int bonus = value <= alpha ? -stat_malus(newDepth)
                           : value >= beta  ? stat_bonus(newDepth)
                                            : 0;
 
@@ -1534,6 +1537,7 @@ void update_all_stats(const Position& pos,
     PieceType              captured;
 
     int quietMoveBonus = stat_bonus(depth + 1);
+    int quietMoveMalus = stat_malus(depth + 1);
 
     if (!pos.capture(bestMove))
     {
@@ -1545,15 +1549,18 @@ void update_all_stats(const Position& pos,
         thisThread->pawnHistory[pawn_structure(pos)][moved_piece][to_sq(bestMove)]
           << quietMoveBonus;
 
+        int moveMalus = bestValue > beta + 97 ? quietMoveMalus      // larger malus
+                                              : stat_malus(depth);  // smaller malus
+
         // Decrease stats for all non-best quiet moves
         for (int i = 0; i < quietCount; ++i)
         {
             thisThread->pawnHistory[pawn_structure(pos)][pos.moved_piece(quietsSearched[i])]
                                    [to_sq(quietsSearched[i])]
-              << -bestMoveBonus;
-            thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bestMoveBonus;
+              << -moveMalus;
+            thisThread->mainHistory[us][from_to(quietsSearched[i])] << -moveMalus;
             update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]),
-                                          to_sq(quietsSearched[i]), -bestMoveBonus);
+                                          to_sq(quietsSearched[i]), -moveMalus);
         }
     }
     else
@@ -1569,14 +1576,14 @@ void update_all_stats(const Position& pos,
         && ((ss - 1)->moveCount == 1 + (ss - 1)->ttHit
             || ((ss - 1)->currentMove == (ss - 1)->killers[0]))
         && !pos.captured_piece())
-        update_continuation_histories(ss - 1, pos.piece_on(prevSq), prevSq, -quietMoveBonus);
+        update_continuation_histories(ss - 1, pos.piece_on(prevSq), prevSq, -quietMoveMalus);
 
     // Decrease stats for all non-best capture moves
     for (int i = 0; i < captureCount; ++i)
     {
         moved_piece = pos.moved_piece(capturesSearched[i]);
         captured    = type_of(pos.piece_on(to_sq(capturesSearched[i])));
-        captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -quietMoveBonus;
+        captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -quietMoveMalus;
     }
 }